In recent years, digital cameras have been becoming a main stream of cameras. In a digital camera including an image-pickup device such as a CCD (Charge Coupled Device) or a CMOS (Complementary Mental Oxide Semiconductor), an image (hereinafter, referred to as taken image) produced by the image pickup device is displayed on a monitor such as an LCD (Liquid Crystal Display) device employed in the digital camera so as to allow the user to confirm the image. Then, an image signal of the taken image is digitalized before being transferred to typically an image processing section employed in a personal computer (PC) through a recording medium such as a flash memory. As an alternative, the taken image is transmitted to a PC by a wired communication using a cable or a radio communication using an infrared ray or the like. In the personal computer serving as a destination of the transmission of the taken image, the image taken by the digital camera is displayed on a monitor such as a CRT (Cathode Ray Tube) or LCD (Liquid Crystal Display) display section of the personal computer so as to allow the user to verify or edit the taken image.
If a digital camera is used to take a picture of a photographing object, which is not sufficiently clear, for example in a shadow area, at a place with no enough sunlight, in a slightly dark room or at a similar location, it is necessary to decrease the speed of a shutter employed in the digital camera or lengthen the exposure time of the photographing object in order to provide enough exposure of the photographing object to the digital camera.
In an image-pickup operation with such a long exposure time, the digital camera is typically fixed on a tripod or the like so as to prevent the digital camera from shaking or trembling. Thus, it is possible to obtain a taken image free of blurring and with proper exposure according to the brightness of the photographing object. When the digital camera is used to take an image of a photographing object by for example holding the camera by a hand, however, the camera shakes due to trembling of the hand. Then, if the digital camera trembles while the shutter of the camera is in an opened state or while the photographing object is being exposed to the camera, the resulting taken image is inadvertently a blurring image caused by the trembling of the digital camera. Such a blurring image is referred to as a hand-shaken image or known as an image resulting from camera trembling.
In addition to the technique to decrease the shutter speed or increase the exposure time, as a method to obtain sufficient exposure, there has been proposed a method to give exposure, which is equivalent to exposure provided by the technique to increase the exposure time, by merely summing up a plurality of taken images cumulatively. For more information on this proposed method, refer to documents such as Patent Document 1.
With the method disclosed in Patent Document 1, however, a plurality of taken images is merely summed up cumulatively. Thus, if a hand holding the digital camera trembles as described above, the camera will produce a blurring image as is the case with the technique to increase the exposure time.
As a method for preventing a taken image from blurring or a hand-shaken image from being produced even if a hand holding the digital camera trembles, there has been introduced a method adopted by a digital camera made by for example Canon Corporation. This method is referred to as an IS (Image Stabilizer).
With the Image Stabilizer, a pre-sensor is provided in an optical-system lens as a sensor for detecting trembling or vibration of the digital camera. Then, in accordance with a digital signal representing the detected trembling or vibration of the digital camera, a portion of a lens group serving as a correction optical system is moved in a direction perpendicular to the optical axis so as to refract the ray of light in a direction canceling the trembling of the taken image.
In accordance with the Image Stabilizer, it is possible to suppress image trembling due to shaking of a hand holding the digital camera or due to infinitesimal vibration shaking caused by a blowing wind as shaking of the base of the photographing apparatus. As a result, the user can be provided with a sharp taken image.
With the Image Stabilizer, however, it is necessary to provide a dedicated sensor for detecting trembling and a mechanism for moving a portion of a lens group serving as a correction optical system at a high speed. Thus, the Image Stabilizer raises a problem of a complicated structure of the digital camera and a problem of a high cost to manufacture the camera.
As another method for avoiding a hand-shaken image, there is known a method by which shift quantities of the second and all subsequent ones among a plurality of taken images are each detected as the quantity of a shift from the first taken image, the positions of the second and all subsequent taken images are then corrected by their respective quantities of the shifts and, finally, the shifted second and all subsequent taken images are sequentially added to the first taken image. For more information on this method, the reader is suggested to refer to documents such as Patent Documents 2, 3, 4, 5, 6, 7 and 8. In accordance with the method disclosed in Patent Documents 2, 3, 4, 5, 6, 7 and 8, an interpolated image having a data array identical with the first taken image is created by interpolation based on each of the second and all subsequent taken images completing a process to correct the positions of the images and the interpolated image is then simply added to the first taken image in pixel units.
In accordance with the method disclosed in Patent Documents 2, 3, 4, 5, 6, 7 and 8, since images are taken consecutively at a high speed and, hence, at a short exposure time, the amount of blurring is small even though a dark picture is resulted in. In order to solve the problem of a dark picture, an image created by interpolation based on the second and all subsequent taken images is added to the first taken image to result in a finally obtained image having brightness similar to an image taken with a proper exposure time.
In the process to create an image created by interpolation based on the second and all subsequent taken images in accordance with the method described in Patent Documents 2 to 8, chrominance signals (or color data) are subjected to an interpolation process adopting an interpolation method using a variety of interpolation functions such as the linear interpolation and the Bi-Cubic interpolation. The chrominance signals include an R (Red) signal (representing red-color data), a G (Green) signal (representing green-color data) and a B (Blue) signal (representing blue-color data), which pertain to one pixel.
[Patent Document 1]    Japanese Patent Laid-open No. Hei 05-236442
[Patent Document 2]    Japanese Patent Laid-open No. 2000-217032
[Patent Document 3]    Japanese Patent Laid-open No. 2000-224460
[Patent Document 4]    Japanese Patent Laid-open No. 2000-244803
[Patent Document 5]    Japanese Patent Laid-open No. 2000-244797
[Patent Document 6]    Japanese Patent Laid-open No. 2000-069352
[Patent Document 7]    Japanese Patent Laid-open No. Hei 10-341367
[Patent Document 8]    Japanese Patent Laid-open No. Hei 09-261526